Optical beam scanning apparatus, optical beam scanning method and image forming apparatus

ABSTRACT

An optical beam scanning apparatus according to the present invention includes a light source, a pre-deflection optical system, a light deflecting device, a post-deflection optical system configured to at least include one or plural first optical elements which act on the luminous fluxes for all color components, plural second optical elements which respectively act on the luminous fluxes for each of color components, and plural first reflection mirrors that are respectively provided on an upstream side of the plural second optical elements in optical paths and reflect luminous fluxes emitted from the first optical elements, and a position adjusting mechanism configured to adjust positions of the first reflection mirrors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from:U.S. provisional application 60/971,540, filed on Sep. 11, 2007, theentire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an optical beam scanning apparatus, anoptical beam scanning method and an image forming apparatus includingthe optical beam scanning apparatus. In particular, the inventionrelates to an optical beam scanning apparatus that can form pluralscanning lines by separating one or plural luminous fluxes, which areemitted from one or plural light sources, in a sub-scanning directionfor each of color components using deflection surface of a deflectingdevice and then, imaging the luminous fluxes by a post-deflectionoptical system and an image forming apparatus including the optical beamscanning apparatus.

BACKGROUND

An image forming apparatus of an electrophotographic system such as alaser printer, a digital copying machine, or a laser facsimile includesan optical beam scanning apparatus that forms an electrostatic latentimage on a photoconductive drum by irradiating a laser beam (a luminousflux) on the surface of a photoconductive drum and scanning the laserbeam.

Recently, besides a monochrome machine including a scanning opticalsystem that uses a single light source, a tandem color machine isproposed. For the tandem color machine, a method of increasing thenumber of laser beams scanned at a time by providing plural lightsources (laser diodes) in one laser unit (a multi-beam method) isproposed for the purpose of realizing an increase in speed of scanningon the surface of a photoconductive drum. In the multi-beam method,plural beams for each of color components (e.g., yellow, magenta, cyan,and black) emitted from the respective light sources are subjected toprocessing in a pre-deflection optical system and are changed to onebeam and made incident on a polygon mirror. The beam deflected by thepolygon mirror is, after passing through an fθ lens configuring apost-deflection optical system, separated into beams for each of thecolor components and irradiated on a photoconductive drum for each ofthe color components.

There is also proposed a color image forming apparatus including anoptical beam scanning apparatus that forms plural scanning lines byseparating one or plural luminous fluxes, which are emitted from one orplural light sources, for each of color components using pluraldeflection surfaces having different angles with respect to a rotationcenter axis of a deflecting device (a polygon mirror) and then, imagingthe luminous fluxes with a post-deflection optical system. For example,according to JP-A-7-256926, there is known a technique for making pluralbeams incident on an identical surface of one polygon mirror, separatingthe beams in order of height in a sub-scanning direction after the beamspass through one set of fθ lenses, and reflecting the separated beams.

Moreover, there is also recently proposed an optical beam scanningapparatus in which an individual focusing lens is provided for each ofcolor components in a post-deflection optical system in order to improveoptical accuracy in a scanning optical system in the optical beamscanning apparatus. For example, according to JP-A-2003-5113, there isknown a technique for making plural beams on an identical surface of onepolygon mirror, separating the beams in order of height in asub-scanning direction after the beams pass through a shared fθ lens,and making the separated beams incident on another fθ lens afterreflecting the beams.

Conventionally, in the optical beam scanning apparatus in which theindividual focusing lens is provided for each of the color components inthe post-deflection optical system, positions of individual focusinglenses included in the post-deflection optical system and other opticalelements are fixed. Therefore, when defocus in a main scanning directionor a sub-scanning direction occurs with respect to optical elementsincluding the individual focusing lenses provided in the post-deflectionoptical system because of an error during manufacturing of a housing,the optical elements including the individual focusing lenses providedin the post-deflection optical system cannot be adjusted and it isdifficult to obtain optical characteristics as designed.

SUMMARY

The present invention has been devised in view of such circumstances andit is an object of the present invention to provide an optical beamscanning apparatus that can suitably adjust optical elements included ina post-deflection optical system in which an individual focusing lens isprovided for each of color components and an image forming apparatusincluding the optical beam scanning apparatus.

In order to solve the problems, an optical beam scanning apparatusaccording to an aspect of the present invention includes a light sourceconfigured to emit one or plural luminous fluxes, a pre-deflectionoptical system configured to form the luminous fluxes emitted from thelight source to image the luminous fluxes as a line image in a directioncorresponding to a main scanning direction, a light deflecting deviceconfigured to scan the luminous fluxes against a scanning object in themain scanning direction, a post-deflection optical system configured toat least include one or plural first optical elements which act on theluminous fluxes for all color components, plural second optical elementswhich respectively act on the luminous fluxes for each of colorcomponents, and plural first reflection mirrors that are respectivelyprovided on an upstream side of the plural second optical elements inoptical paths and reflect luminous fluxes emitted from the first opticalelements, the post-deflection optical system imaging the luminous fluxesscanned by the light deflecting device on the scanning object, and aposition adjusting mechanism configured to adjust positions of the firstreflection mirrors.

In order to solves the problems, an optical beam scanning methodaccording to another aspect of the present invention includes the stepsof preparing an optical beam scanning apparatus including one or pluralfirst optical element, plural second optical elements and plural firstreflection mirrors, emitting one or plural luminous fluxes, forming theemitted luminous fluxes to image the luminous fluxes as a line image ina direction corresponding to a main scanning direction, separating theluminous fluxes in a sub-scanning direction for each of color componentsand scanning the luminous fluxes against a scanning object in the mainscanning direction, at least, acting on the luminous fluxes for allcolor components by one or plural first optical elements, respectivelyacting on the luminous fluxes for each of color components, and imagingthe scanned luminous fluxes on the scanning object, reflecting luminousfluxes emitted from the first optical elements by plural firstreflection mirrors respectively provided on an upstream side of theplural second optical elements in optical paths, and adjusting positionsof the first reflection mirrors.

In order to solve the problems, an image forming apparatus according toanother aspect of the present invention is an image forming apparatusincluding an optical beam scanning apparatus, the optical beam scanningapparatus including a light source configured to emit one or pluralluminous fluxes, a pre-deflection optical system configured to form theluminous fluxes emitted from the light source to image the luminousfluxes as a line image in a direction corresponding to a main scanningdirection, a light deflecting device configured to scan the luminousfluxes against a scanning object in the main scanning direction, apost-deflection optical system configured to at least include one orplural first optical elements which act on the luminous fluxes for allcolor components, plural second optical elements which respectively acton the luminous fluxes for each of color components, and plural firstreflection mirrors that are respectively provided on an upstream side ofthe plural second optical elements in optical paths and reflect luminousfluxes emitted from the first optical elements, the post-deflectionoptical system imaging the luminous fluxes scanned by the lightdeflecting device on the scanning object, and a position adjustingmechanism configured to adjust positions of the first reflectionmirrors.

In order to solve the problems, an optical beam scanning apparatusaccording to still another aspect of the present invention includes alight source configured to emit one or plural luminous fluxes, apre-deflection optical system configured to form the luminous fluxesemitted from the light source to image the luminous fluxes as a lineimage in a direction corresponding to a main scanning direction, a lightdeflecting device configured to scan the luminous fluxes against ascanning object in the main scanning direction, a post-deflectionoptical system configured to at least include one or plural firstoptical elements which act on the luminous fluxes for all colorcomponents, and plural second optical elements which respectively act onthe luminous fluxes for each of color components, the post-deflectionoptical system imaging the luminous fluxes scanned by the lightdeflecting device on the scanning object, and a position adjustingmechanism configured to adjust positions of the second optical elements.

In order to solves the problems, an optical beam scanning methodaccording to still another aspect of the present invention includes thesteps of preparing an optical beam scanning apparatus including one orplural first optical element and plural second optical elements,emitting one or plural luminous fluxes, forming the emitted luminousfluxes to image the luminous fluxes as a line image in a directioncorresponding to a main scanning direction, scanning the luminous fluxesagainst a scanning object in the main scanning direction, at least,acting on the luminous fluxes for all color components by one or pluralfirst optical elements, respectively acting on the luminous fluxes foreach of color components, and imaging the scanned luminous fluxes on thescanning object, and adjusting positions of the second optical elements.

In order to solve the problems, an image forming apparatus according tostill another aspect of the present invention is an image formingapparatus including an optical beam scanning apparatus, the optical beamscanning apparatus including a light source configured to emit one orplural luminous fluxes, a pre-deflection optical system configured toform the luminous fluxes emitted from the light source to image theluminous fluxes as a line image in a direction corresponding to a mainscanning direction, a light deflecting device configured to scan theluminous fluxes against a scanning object in the main scanningdirection, a post-deflection optical system configured to at leastinclude one or plural first optical elements which act on the luminousfluxes for all color components, and plural second optical elementswhich respectively act on the luminous fluxes for each of colorcomponents, the post-deflection optical system imaging the luminousfluxes scanned by the light deflecting device on the scanning object,and a position adjusting mechanism configured to adjust positions of thesecond optical elements.

DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a side view showing a configuration of an image formingapparatus having an optical beam scanning apparatus to which the presentinvention is applied;

FIGS. 2A to 2C are diagrams showing expansion of reflecting by areflection mirror provided in the optical beam scanning apparatus;

FIG. 3 is an explanatory diagram for explaining a shape of a lightblocking plate that blocks optical paths of beams LY, LM, and LC to asurface discrimination sensor arranged on a downstream side of theoptical path with respect to optical elements;

FIGS. 4A to 4F are a plan view, a sectional view, and side views of apolygon mirror main body of a deflecting device used in a scanningoptical system of the optical beam scanning apparatus;

FIG. 5 is a diagram showing a state in which a reflection surface (Ysurface) that reflects beams LY, LM, LC, and LK tilts in a directioncloser to a rotation axis direction;

FIG. 6 is a diagram showing a detailed configuration of the optical beamscanning apparatus shown in FIG. 1;

FIGS. 7A and 7B are diagrams showing a position adjusting mechanism thatadjusts positions of reflection mirrors on an upstream side of opticalpaths of individual lenses;

FIGS. 8A and 8B are diagrams showing a direction in which the positionsof the reflection mirrors on the upstream side of the optical paths ofthe individual lenses are adjusted; and

FIGS. 9A and 9B are diagrams showing a position adjusting mechanism thatadjusts positions of the individual lenses.

DETAILED DESCRIPTION

An embodiment of the present invention is explained below with referenceto the accompanying drawings.

FIG. 1 is a diagram showing a configuration of an image formingapparatus 1 having an optical beam scanning apparatus 3 according to anembodiment of the present invention. In the explanation of thisembodiment, the image forming apparatus 1 is applied to a color printer.However, the application of the image forming apparatus 1 is not limitedto this. The image forming apparatus 1 can also be applied to variousimage output apparatuses such as a full-color copying apparatus, afacsimile apparatus, and a workstation apparatus.

The image forming apparatus 1 includes the optical beam scanningapparatus (an exposing apparatus) 3 that generates image lightcorresponding to an image signal and an image forming unit thattransfers a toner image visualized by a toner as a developer onto paperP as a transfer medium used for output, which is called hard copy orprintout, on the basis of the image light supplied by the optical beamscanning apparatus 3 and outputs the toner image. Every time the tonerimage is formed, the paper P is fed to the image forming unit from apaper holding unit 7 that holds an arbitrary number of sheet-like piecesof paper P having a predetermined size and can feed the pieces of paperP one by one according to timing when the toner image is formed in theimage forming unit.

A conveying path 9 that guides the paper P from the paper holding unit 7to the image forming unit is provided between the paper holding unit 7and the image forming unit. The conveying path 9 guides the paper P to afixing device 11 that fixes, on the paper P, the toner image transferredonto the paper P through a transfer device 9A that transfers the tonerimage formed in the image forming apparatus. As another function, theconveying path 9 guides the paper P having the toner image fixed thereonby the fixing device 11 to an image-output holding unit la also servingas a part of a cover that covers the image forming unit.

The image forming unit has an intermediate transfer belt 13 obtained byforming an insulative film having predetermined thickness in an endlessbelt shape. A belt obtained by forming metal in a thin sheet shape and,then, protecting the surface thereof with resin may be applied as theintermediate transfer belt 13. Predetermined tension is applied to theintermediate transfer belt 13 by a driving roller 15, a first tensionroller 17 a and a second tension roller 17 a, and a transfer roller 19.An arbitrary position of the intermediate transfer belt 13 parallel toan axis of the driving roller 15 moves in an arrow A direction when thedriving roller 15 is rotated. In other words, a belt surface of theintermediate transfer belt 13 turns in one direction at speed ofmovement of an outer peripheral surface of the driving roller 15.

First to fourth image forming units 21Y, 21M, 21C, and 21K are arrayedat predetermined intervals in a section in which the belt surface of theintermediate transfer belt 13 moves substantially flat with thepredetermined tension applied thereto by the respective rollers (thedriving roller 15, the first tension roller 17 a and the second tensionroller 17 b, and the transfer roller 19).

The first to fourth image forming units 21Y, 21M, 21C, and 21Krespectively include at least developing devices 22Y, 22M, 22C, and 22Kin which toners of arbitrary colors of Y (yellow), M (magenta), C(cyan), and BK (black) are stored and photoconductive drums 23Y, 23M,23C, and 23K that hold electrostatic latent images developed by therespective developing devices 22 (the developing devices 22Y, 22M, 22C,and 22K). Electrostatic latent images corresponding to images of colorsdeveloped by the developing devices 22Y, 22M, 22C, and 22K provided inthe respective image forming units 21 are formed, by image light fromthe optical light scanning apparatus 3, on the surfaces (outerperipheral surfaces) of the photoconductive drums 23Y, 23M, 23C, and 23Kincluded in the respective image forming units 21. Consequently, thetoners are selectively supplied by any one of the developing devices22Y, 22M, 22C, and 22K corresponding to the electrostatic latent images.As a result, toner images of predetermined colors are formed on thephotoconductive drums 23Y, 23M, 23C, and 23K, respectively.

In the first to fourth image forming units 21Y, 21M, 21C, and 21K,transfer rollers 31Y, 31M, 31C, and 31K for transferring the tonerimages held by the respective photoconductive drums 23 onto theintermediate transfer belt 13 are respectively provided in positionsopposed to the photoconductive drums 23Y, 23M, 23C, and 23K via theintermediate transfer belt 13. The transfer rollers 31Y, 31M, 31C, and31K are provided on a rear side of the intermediate transfer belt 13.

A not-shown image-signal supplying unit is provided in the image formingapparatus 1 in which the developing devices 22 (22Y, 22M, 22C, and 22K),the photoconductive drums 23 (23Y, 23M, 23C, and 23K), and the transferrollers 31 (31Y, 31M, 31C, and 31K) are arrayed as described above. Theimage-signal supplying unit supplies an image signal for each of colorcomponents to the optical beam scanning apparatus 3. The optical beamscanning apparatus 3 generates image light corresponding to the imagesignal supplied from the image-signal supplying unit and irradiates thegenerated image light on the surfaces of the photoconductive drums 23(23Y, 23M, 23C, and 23K) integral with the developing devices 22 (22Y,22M, 22C, and 22K) that hold the toners of the color componentscorresponding to the image light. At this point, the respective imageforming units 21 form electrostatic latent images at predeterminedtiming such that the sequentially-transferred toner images aresuperimposed one on top of another on the intermediate transfer belt 13.The electrostatic latent images are developed (visualized) by thedeveloping devices 22 corresponding to the image forming units 21.

The toner images formed on the photoconductive drums 23 of therespective image forming units 21 are transferred onto the intermediatetransfer belt 13 by the transfer rollers 31 (31Y, 31M, 31C, and 31K) asprimary transfer devices corresponding to the respective photoconductivedrums 23 (23Y, 23M, 23C, and 23K). At this point, the toner images of Y,M, C, and BK are sequentially stacked on the intermediate transfer belt13 that moves at predetermined speed. In the case of FIG. 1, rollerbodies are used as the transfer rollers 31 serving as the primarytransfer devices. However, the transfer rollers 31 are not limited tothe roller bodies and may be voltage generating devices such asscorotrons.

A secondary transfer roller 81 as a secondary transfer device isprovided in the image forming apparatus 1. The secondary transfer roller81 comes into contact with the intermediate transfer belt 13 atpredetermined pressure in a transfer position 9A of the conveying path9. The secondary transfer roller 81 as the secondary transfer devicetransfers a full-color toner image formed on the intermediate transferbelt 13 onto the paper P guided to the transfer position 9A of theconveying path 9.

Registration rollers 61 that temporarily stop the paper P, which isguided from the paper holding unit 7 to the transfer position 9A, isprovided in a predetermined position in the conveying path 9 between thepaper holding unit 7 and the transfer position 9A. The registrationrollers 61 include two rollers. At least one roller rotates in apredetermined direction and the other roller is pressed against oneroller at predetermined pressure via a not-shown press-contactmechanism.

The paper P is guided through the conveying path 9 from the paperholding unit 7 to the transfer position 9A and temporarily stopped bythe registration rollers 61. This makes it possible to correct a tilt (atile of the paper P with respect to a conveying direction) that canoccur during conveyance through the conveying path 9 from the paperholding unit 7 to the transfer position 9A.

According to timing when the registration roller 61 is rotated again,timing when a toner image carried to the transfer position 9A accordingto the movement of the belt surface of the intermediate transfer belt 13reaches the transfer position 9A and timing when the paper P reaches thetransfer position 9A are set. This makes it possible to arbitrarily seta position of the toner image with respect to the paper P and manage theposition of the toner image with respect to the paper P.

FIGS. 2A to 2C are diagrams showing expansion of reflecting by thereflection mirror provided in the optical beam scanning apparatus 3.FIG. 2A is a diagram viewed from an arrow X direction in FIG. 2B. FIG.2C is a diagram viewed from an arrow Y direction in FIG. 2B. The opticalbeam scanning apparatus 3 includes, as shown in FIGS. 2A to 2C, at leasta light source (a semiconductor laser) 33 that outputs image light(exposure light), a deflecting device 35 that scans the image light fromthe light source 33 in a raster direction for output (hard copy orprintout) and guides a beam to the respective photoconductive drums 23arranged at predetermined intervals in the sub-scanning direction, apost-deflection optical system (an image forming optical system) 37 thatfocuses the image light, which is raster-deflected (scanned) by thedeflecting device 35, on the photoconductive drums 23 (23Y, 23M, 23C,and 23K) of the first to fourth image forming units 21 under apredetermined condition regardless of a deflection angle, and apre-deflection optical system (an exposure light shaping optical system)39 that guides the image light from the light source 33 to thedeflecting device 35 under a predetermined condition.

A direction in which respective laser beams are deflected (scanned) bythe deflecting device 35 (a rotation axis direction of thephotoconductive drums 23) is defined as “main scanning direction” and adirection perpendicular to the optical axis of the optical system andthe main scanning direction is defined as “sub-scanning direction”.Therefore, the sub-scanning direction is a drum rotating direction onthe photoconductive drum 23.

The deflecting device 35 includes a polygon mirror main body (aso-called polygon mirror) in which, for example, eight plane reflectionsurfaces (plane reflection mirrors) are arranged in a regular polygonalshape and a motor that rotates the polygon mirror main body in the mainscanning direction at predetermined speed. The polygon mirror main bodyis a rotatable reflection element and fixed to a shaft of the motor. Thenumber of reflection surfaces provided in the polygon mirror main bodyas the reflection element and the number of revolutions are setaccording to requirements of output (i.e., resolution and output speedrequired of the image forming apparatus 1 and other requirements) Thereflection surfaces (polygon mirror surfaces) of the deflecting device35 have required angles with respect to a rotation center axis of thepolygon mirror main body such that a beam can be guided to a scanningline position where electrostatic latent images are formed on therespective photoconductive drums 23.

The post-deflection optical system 37 includes at least a shared lens37-1 used for all scanning lines for forming electrostatic latent imagesof the respective colors guided to the respective photoconductive drums23 and an individual lens 37 corresponding to each of the scanning linesfor forming electrostatic latent images of the respective colors guidedto the respective photoconductive drums 23. The shared lens 37-1 givesdifferent light focusing properties to the image light raster-scanned bythe deflecting device 35 according to positions in a longitudinaldirection of the respective photoconductive drums 23Y, 23M, 23C, and 23K(i.e., positions on the photoconductive drums 23 that depend on swingangles (deflection angles) of image light caused by raster deflection ofthe image light in the main scanning direction orthogonal to a directionin which the paper P is conveyed (a direction in which thephotoconductive drums 23 are rotated)). The shared lens 37-1 has aslender shape extending in the longitudinal direction of thephotoconductive drums 23.

The post-deflection optical system 37 includes, besides the shared lens37-1 and the individual lens 37-2, various optical elements (e.g., amirror and a filter) for guiding the image light raster-scanned by thedeflecting device 35 to the respective photoconductive drums 23Y, 23M,23C, and 23K of the first to fourth image forming units 21. The sharedlens 37-1 and the individual lens 37-2 may be replaced with mirrorshaving curved surfaces similar to those of these lenses by optimizingtypes and shapes of optical elements and combining arrays. Thereplacement with mirrors may be applied to both the shared lens 37-1 andthe individual lens 37-2 or may be applied to only one of the lenses.

A focus position (a focus position on a front side in the sub-scanningdirection) of the shared lens 37-1 is set further on an upstream side (aside where the rotation center axis of the polygon mirror main body ispresent; the upstream side may extend beyond the rotation center axis)than the reflection surface (the polygon mirror surface) of thedeflecting device 35 such that an inter-beam distance of beams emittedfrom the shared lens 37-1 for generating electrostatic latent images ofthe respective colors increases toward downstream of the optical paths.

The pre-deflection optical system 39 forms the image light from thelight source 33 such that the image light is formed in (focused in) asectional beam shape that satisfies a predetermined condition when theimage light is raster-scanned by the deflecting device 35 and condensedin predetermined positions in the longitudinal direction of therespective photoconductive drums 23Y, 23M, 23C, and 23K in thepost-deflection optical system 37. The pre-deflection optical system 39includes optical elements such as a condenser lens, a mirror, and anaperture.

Predetermined intervals corresponding to positions where the respectiveimage forming units 21 are arrayed (substantially equal intervals on thebelt surface of the intermediate transfer belt 13) are given to theimage light emitted from the optical beam scanning apparatus 3.Intervals of the image light emitted from the optical beam scanningapparatus 3 are defined to integer times as large as a circumference (arotation pitch of the driving roller 15) obtained by adding up thediameter of the driving roller 15 and the thickness of the intermediatetransfer belt 13. Therefore, even if there is eccentricity or the likein the driving roller 15, since the same period is given when images areformed in the first to fourth image forming units 21, it is possible toreduce the influence of the eccentricity such as color drift.

The scanning optical system of the optical beam scanning apparatus 3includes a surface discrimination sensor 43 that outputs a signal onlywhen the beam LK of BK (black) is scanned on the polygon mirror surfaceand a horizontal synchronizing sensor 44 for determining timing fordrawing an image in the main scanning direction. A beam made incident onthe horizontal synchronizing sensor 44 passes through the shared lens37-1 and, then, passes through an optical element 51. The opticalelement 51 focuses beams passing through the different optical paths onthe horizontal synchronizing sensor 44 in the sub-scanning directionwhile setting heights in the sub-scanning direction of all the opticalpaths substantially identical on the surface of the horizontalsynchronizing sensor 44. The optical element 51 is a convex cylindricallens on a surface on one side (a surface on the downstream side of theoptical paths) thereof in this embodiment. A light blocking plate 52 isarranged on the upstream side of the optical path of the optical element51. As shown in FIG. 3, the light blocking plate 52 has a shape forblocking the optical path of the beams LY, LM, and LC to the surfacediscrimination sensor 43 arranged on the downstream side of the opticalpaths with respect to the optical element 51. The light blocking plate52 causes only the beam LK to pass through the surface discriminationsensor 43 via the optical element 51. In this embodiment, as shown inFIG. 3, the four beams LY, LM, LC, and LK emitted from the shared lens37-1 after being deflected by the deflecting device 35 are arrayed inorder of the beams LC, LK, LM, and LY from the upstream side in thesub-scanning direction to the downstream side in the sub-scanningdirection. However, the array of the beams LC, LK, LM, and LY is notlimited to this. The beams LC, LK, LM, and LY may be arrayed in order ofthe beams LY, LM, LK, and LC from the upstream side in the sub-scanningdirection to the downstream side in the sub-scanning direction.

On the other hand, the light blocking plate 52 causes all the beams LY,LM, LC, and LK to pass through, via the optical element 51, thehorizontal synchronizing sensor 44 arranged on the downstream side ofthe optical paths with respect to the optical element 51. This makes itpossible to suitably adjust, while discriminating, by the surfacediscrimination sensor 43, the black beam among the laser beams of therespective colors guided from the deflecting device 35 via the opticalelement 51, phases of the laser beams of the respective colors for eachof the laser beams. Further, it is possible to prevent occurrence ofcolor drift even in a situation in which there is an error in accuracyof an angle of the deflection surface of the deflecting device 35 and anerror is likely to occur in rotating speed of the deflecting device 35.Moreover, it is possible to prevent occurrence of distortion in an imageof a single color.

It goes without saying that the light blocking plate 52 may cause anyone of the beams LY, LM, and LC to pass through the surfacediscrimination sensor 43 rather than causing only the beam LK to passthrough the surface discrimination sensor 43. In this embodiment, theoptical element 51 is provided in the optical paths between the opticalelement 37-1 that acts on the luminous fluxes deflected by all thedeflection surfaces of the deflecting device 35 and the horizontalsynchronizing sensor 44. However, the optical element 51 may be providedin the optical paths between the deflecting device 35 and the horizontalsynchronizing sensor 44.

FIGS. 4A to 4F are a plan view, a sectional view, and side view of thepolygon mirror main body of the deflecting device 35 used in thescanning optical system of the optical beam scanning apparatus 3. FIG.4A is a plan view of the polygon mirror main body of the deflectingdevice 35. FIG. 4B is a sectional view of the polygon mirror main bodyof the deflecting device 35. FIGS. 4C to 4F are side views of thepolygon mirror main body of the deflecting device 35 viewed from apredetermined direction.

The sectional view of the polygon mirror main body of the deflectingdevice 35 shown in FIG. 4B shows a reference surface in setting a tiltof the reflection surfaces of the polygon mirror main body. A motor isprovided on an A side of the reference surface via a not-shown shaft. Asshown in FIGS. 4C to 4F, the reflection surfaces of the polygon mirrormain body (the polygon mirror) have required tilts with respect to therotation center axis (a rotation center axis of the motor, in otherwords, a hole center axis of the polygon mirror main body). Absolutevalues of the tilts of the reflection surfaces are maximum and equal atθ₁ and θ₃ and signs of the tilts are set opposite. The tilts have arelation of θ₁=−θ₃ and have a relation of θ₁>θ₂>θ₄>θ₃ or θ₁<θ₂<θ₄<θ₃.For example, when a value of θ is a minus numerical value, this meansthat the reflection surface tilts in a direction closer to a rotationaxis direction as the reflection surface is further away from thereference surface A. When a value of θ is a plus numerical value, thismeans that the reflection surface tilts in the direction closer to therotation axis direction as the reflection surface is further away from asurface on the opposite side of the reference surface A. Specifically,for example, as shown in FIG. 5, the reflection surface (C surface) thatreflects the beam LC of the color component C and the reflection surface(K surface) that reflects the beam LK of the color component K tilt inthe direction closer to the rotation axis direction as the reflectionsurfaces are further away from the surface on the reference surface A.The reflection surface (M surface) that reflects the beam LM of thecolor component M and the reflection surface (Y surface) that reflectsthe beam LY of the color component Y tilt in the direction away from therotation axis direction as the reflection surfaces are further away fromthe reference surface A.

By arranging the reflection surfaces in this way, it is possible tocontrol a maximum value of a tilt angle of the reflection surfaces ofthe polygon mirror main body to be as small as possible compared withother those in other arrangements. Since deterioration in a focusingcharacteristic increases as the tilts of the reflection surfaces of thepolygon mirror main body increase, it is possible to suitably controlthe deterioration in the focusing characteristic.

In FIG. 6, optical paths for guiding beams to the respectivephotoconductive drums 23Y, 23M, 23C, and 23K using two reflectionmirrors 40 and 41 for the three colors of Y, M, and C and using onereflection mirror 42 for one color of BK are shown. As described above,the focus position (the focus position on the front side in thesub-scanning direction) of the shared lens 37-1 is set further on theupstream side (the side where the rotation center axis of the polygonmirror main body is present; the upstream side may extend beyond therotation center axis) than the reflection surface (the polygon mirrorsurface) of the deflecting device 35 such that an inter-beam distance ofbeams emitted from the shared lens 37-1 for generating electrostaticlatent images of the respective colors increases toward downstream ofthe optical paths. Consequently, beams further on the upstream side ofthe optical paths in reflection mirrors 40Y, 40M, 40C, and 42K forseparating, for each of the color components, the beams raster-deflectedby the deflecting device 35 have wider intervals in the same position ina beam traveling direction. The four reflection mirrors are arranged inorder of the reflection mirrors 40Y, 40M, 40C, and 42K in order from theupstream side. The intervals in the same position in the beam travelingdirection are in a relation of LY−LM>LM−LK>LK−LC.

In optical paths of beams reflected by the sets of the two reflectionmirrors 40 and 41, individual lenses 37-2 (37-2Y, 37-2M, and 37-2C) arearranged between the sets of the two reflection mirrors 40 and 41,respectively. On the other hand, in an optical path of a beam reflectedby the one reflection mirror 42K, an individual lens 37-2K is arrangedafter the reflection mirror 42K is arranged. In this embodiment, thebeams LC and LY are beams at both the ends in the sub-scanningdirection. The beam LY at one end in the sub-scanning direction isreflected by the reflection mirror 40Y on the most upstream side. Thebeam LC at the other end in the sub-scanning direction is reflected bythe reflection mirror 40C second from the most downstream side. Thereflection mirror 40C is chamfered in advance not to block the opticalpath of the beam L.

Conventionally, in the optical beam scanning apparatus 3 in which theindividual lens 37-2 is provided for each of the color components in thepost-deflection optical system 37, positions of the individual lenses37-2 and the other optical elements included in the post-deflectionoptical system 37 are fixed. Therefore, when defocus in the mainscanning direction or the sub-scanning direction occurs with respect tothe optical elements including the individual lenses 37-2 provided inthe post-deflection optical system 37 because of an error duringmanufacturing of the housing, the optical elements including theindividual lenses 37-2 provided in the post-deflection optical system 37cannot be adjusted and it is difficult to obtain optical characteristicsas designed. Therefore, first, in order to correct defocus in the mainscanning direction with respect to the optical elements including theindividual lenses 37-2 and the like included in the post-deflectionoptical system 37 caused by an error during manufacturing of thehousing, a position adjusting mechanism 70 for adjusting positions ofthe reflection mirrors 40Y, 40M, and 40C arranged on the upstream sideof the individual lens 37-2 provided for each of the color components inthe post-deflection optical system 37 is provided. When defocus in themain scanning direction with respect to the optical elements occurs, thepositions of the reflection mirrors 40Y, 40M, and 40C are adjusted asappropriate. This makes it possible to suitably adjust the opticalelements included in the post-deflection optical system 37 in which theindividual focusing lens is provided for each of the color components.The position adjusting mechanism 70 is explained below. The positionadjusting mechanism 70 that adjusts a position of each of the reflectionmirrors 40Y, 40M, and 40C is provided in the optical beam scanningapparatus 3.

FIGS. 7A and 7B show the position adjusting mechanism 70 for adjusting aposition of, for example, the reflection mirror 40Y among the reflectionmirrors 40Y, 40M, and 40C. FIG. 7A is a side view of one end of theposition adjusting mechanism 70. FIG. 7B is a side view of the other endof the position adjusting mechanism 70. As shown in FIG. 7A, thereflection mirror 40Y is pressed against and fixed to a sheet metal 48via a blade spring 45 at points P and Q. The blade spring 45 is screwedto the sheet metal 48 by a screw 56-1. An alternate long and two shortdashes line 46 a shown in FIG. 7A is a line connecting points at anequal distance from an upper edge of an optical path (an incidentoptical path) of the beam LY to the reflection mirror 40Y and a loweredge of an optical path (an incident optical path) of the beam LMadjacent to the optical path of the beam LY. Specifically, as shown inFIGS. 8A and 8B, the optical path (the incident optical path) of thebeam LY to the reflection mirror 40Y is defined by an upper edge α1 anda lower edge α2 of the beam LY. The optical path (the incident opticalpath) of the beam LM to the reflection mirror 40M is defined by an upperedge β1 and a lower edge β2 of the beam LM. The alternate long and twoshort dashes line 46 a shown in FIG. 7A is a line (a bisector)connecting points at an equal distance from the upper edge α1 of theoptical path (the incident optical path) of the beam LY to thereflection mirror 40Y and the lower edge β2 of the optical path (theincident optical path) of the beam LM. In this embodiment, thereflection mirrors 40Y, 40M, and 40C are moved by a predetermineddistance in parallel to the alternate long and two short dashes line 46a shown in FIGS. 7A and 7B and 8A and 8B to adjust focuses in the mainscanning direction of the beams LY, LM, and LC. This is for the purposeof, for example, preventing the beam LY from blocking the optical pathof the beam LM even if adjustment of the focus in the main scanningdirection of the beam LY is performed and preventing the beam LY fromfalling on an edge section of the reflection mirror 40Y. In the case ofthe position adjusting mechanism 70 for adjusting a position of thereflection mirror 40M other than the reflection mirror 40Y, thealternate long and two short dashes line 46 a is a line (a bisector)connecting points at an equal distance from the upper edge α1 of theoptical path (the incident optical path) of the beam LM to thereflection mirror 40M and the lower edge β2 of the optical path (theincident optical path) of the beam LK.

Two protrusions 47-1 and 47-2 on an optical housing used for adjusting aposition of the reflection mirror 40Y are inserted into long holes 57-1and 57-2 provided in the sheet metal 48 and are, then, provided on analternate long and two short dashes line 46 b parallel to the alternatelong and two short dashes line 46 a. Consequently, on a side at one endof the position adjusting mechanism 70 shown in FIG. 7A, the sheet metal48 is movable parallel to the alternate long and two short dashes line46 a. Therefore, when the sheet metal 48 is moved by a predetermineddistance in parallel to the alternate long and two short dashes line 46a, according to the movement of the sheet metal 48, the reflectionmirror 41Y fixed to the sheet metal 48 also moves in parallel to thealternate long and two short dashes line 46 a by a movement amountidentical with that of the sheet metal 48.

On the other hand, as shown in FIG. 7B, the reflection mirror 40Y ispressed against and fixed to the sheet metal 48 via the blade spring 45at a point R on the other end side. The blade spring 45 is screwed tothe sheet metal 48 by a screw 56-3. Two protrusions 47-3 and 47-4 on theoptical housing used for adjusting a position of the reflection mirror40Y are inserted into long holes 57-3 and 57-4 provided in the sheetmetal 48 and are, then, provided on the alternate long and two shortdashes line 46 b parallel to the alternate long and two short dashesline 46 a. Consequently, the sheet metal 48 is also movable in parallelto the alternate long and two short dashes line 46 a on a side at theother end of the position adjusting mechanism 70 shown in FIG. 7A.

When defocus in the main scanning direction with respect to the opticalelements including the individual lenses 37-2 included in thepost-deflection optical system 37 is corrected, at first, a finite lens53 of the pre-deflection optical system 39 is moved in an optical axisdirection to adjust a focus in the main scanning direction of the beamLK and, then, the pre-deflection optical system 39 including the finitelens 53 is bonded and fixed to the housing. Subsequently, the reflectionmirrors 40Y, 40M, and 40C are moved by a predetermined distance inparallel to the alternate long and two short dashes line 46 a shown inFIGS. 7A and 7B and 8A and 8B to adjust focuses in the main scanningdirection of the beams LY, LM, and LC. Consequently, opticalcharacteristics as designed can be obtained. In this embodiment, whenfocuses in the main scanning direction of the beams LY, LM, LC, and LKare adjusted, optical path length is changed further on the upstreamside than the individual lenses 37-2. Therefore, it is possible tosuppress the influence on defocus in the sub-scanning direction involvedin the focus adjustment in the main scanning direction of the beams.

A position adjusting mechanism 71 that moves the individual lenses37-2Y, 37-2M, and 37-2C in a direction parallel to incident opticalpaths and adjusts the same when defocus in the sub-scanning directionoccurs is provided. The position adjusting mechanism 71 for theindividual lenses 37-2 is explained below. The position adjustingmechanism 71 that adjusts a position of each of the individual lenses37-2Y, 37-2M, and 37-2C is provided in the optical beam scanningapparatus 3.

FIGS. 9A and 9B show the position adjusting mechanism 71 for adjusting aposition of, for example, the individual lens 37-2Y among the individuallenses 37-2Y, 37-2M, and 37-2C. FIG. 9A is a side view of one end of theposition adjusting mechanism 71. FIG. 9B is a side view of the other endof the position adjusting mechanism 71. As shown in FIG. 9A, theindividual lens 37-2Y is pressed against and fixed to a sheet metal 55via a blade spring 72 at points P and Q. The blade spring 72 is screwedto the sheet metal 55 by a screw 73-1. An alternate long and two shortdashes line 74 a shown in FIG. 9A is an incident optical path of thebeam LY.

Two protrusions 75-1 and 75-2 on an optical housing used for adjusting aposition of the individual lens 37-2Y are inserted into long holes 76-1and 76-2 provided in the sheet metal 55 and are, then, provided on analternate long and two short dashes line 74 b parallel to the alternatelong and two short dashes line 74 a. Consequently, on a side at one endof the position adjusting mechanism 71 shown in FIG. 9A, the sheet metal55 is movable parallel to the alternate long and two short dashes line74 a. Therefore, when the sheet metal 55 is moved by a predetermineddistance in parallel to the alternate long and two short dashes line 74a, according to the movement of the sheet metal 55, the individual lens37-2Y fixed to the sheet metal 55 also moves in parallel to thealternate long and two short dashes line 74 a by a movement amountidentical with that of the sheet metal 55.

On the other hand, as shown in FIG. 9B, the individual lens 37-2Y ispressed against and fixed to the sheet metal 55 via the blade spring 72at a point R on the other end side. The blade spring 72 is screwed tothe sheet metal 55 by a screw 73-3. Two protrusions 75-3 and 75-4 on theoptical housing used for adjusting a position of the individual lens37-2Y are inserted into long holes 76-3 and 76-4 provided in the sheetmetal 55 and are, then, provided on the alternate long and two shortdashes line 74 b parallel to the alternate long and two short dashesline 74 a. Consequently, the sheet metal 55 is also movable in parallelto the alternate long and two short dashes line 74 a on a side at theother end of the position adjusting mechanism 71 shown in FIG. 9A.

When defocus in the sub-scanning direction with respect to the opticalelements included in the post-deflection optical system 37 is corrected,at first, a cylindrical lens 54 (a lens having positive power only inthe sub-scanning direction) of the pre-deflection optical system 39 ismoved in an optical axis direction to adjust a focus in the sub-scanningdirection of the beam LK and, then, the pre-deflection optical system 39including the cylindrical lens 54 is bonded and fixed to the housing.Subsequently, the individual lenses 37-2Y, 37-2M, and 37-2C are moved bya predetermined distance in parallel to the alternate long and two shortdashes line 74 a shown in FIGS. 9A and 9B to adjust focuses in thesub-scanning direction of the beams LY, LM, and LC. Consequently,optical characteristics as designed can be obtained.

It is also possible that the position adjusting mechanism 71 foradjusting a position of the individual lens 37-2K is provided, thecylindrical lens 54 of the pre-deflection optical system 39 is fixed tothe housing, and, then the individual lenses 37-2Y, 37-2M, 37-2C, and37-2K are moved by a predetermined distance in parallel to the alternatelong and two short dashes line 74 a shown in FIGS. 9A and 9B to adjustfocuses in the sub-scanning direction of all the beams LY, LM, LC, andLK.

1. An optical beam scanning apparatus comprising: a light sourceconfigured to emit one or plural luminous fluxes; a pre-deflectionoptical system configured to form the luminous fluxes emitted from thelight source to image the luminous fluxes as a line image in a directioncorresponding to a main scanning direction; a light deflecting deviceconfigured to scan the luminous fluxes against a scanning object in themain scanning direction; a post-deflection optical system configured toat least include one or plural first optical elements which act on theluminous fluxes for all color components, plural second optical elementswhich respectively act on the luminous fluxes for each of colorcomponents, and plural first reflection mirrors that are respectivelyprovided on an upstream side of the plural second optical elements inoptical paths and reflect luminous fluxes emitted from the first opticalelements, the post-deflection optical system imaging the luminous fluxesscanned by the light deflecting device on the scanning object; and aposition adjusting mechanism configured to adjust positions of the firstreflection mirrors.
 2. The optical beam scanning apparatus according toclaim 1, wherein the deflection surfaces provided in the lightdeflecting device have different angles with respect to the rotationcenter axis for each of the color components.
 3. The optical beamscanning apparatus according to claim 1, wherein, in the post-deflectionoptical system, plural second reflection mirrors that are respectivelyprovided on a downstream side of the plural second optical elements ofthe optical paths excluding one optical path, in which one of the secondoptical elements is included, and reflect luminous fluxes emitted fromthe second optical elements are further provided; and plural opticalpaths having sets of two reflection mirrors including the firstreflection mirror and the second reflection mirror and one optical pathhaving only one of the first reflection mirrors are provided accordingto the color components.
 4. The optical beam scanning apparatusaccording to claim 1, wherein the position adjusting mechanism adjustspositions of the first reflection mirrors in a direction indicated by aline connecting substantially centers of optical paths adjacent to eachother among plural optical paths corresponding to an angle with respectto the rotation center axis of the light deflecting device.
 5. Theoptical beam scanning apparatus according to claim 4, wherein thedirection indicated by the line connecting substantially the centers ofthe optical paths adjacent to each other is a direction indicated by aline connecting points at a substantially equal distance from an upperedge and a lower edge of beams passing through the optical pathsadjacent to each other.
 6. The optical beam scanning apparatus accordingto claim 1, wherein a third optical element which gives predeterminedcharacteristic to the luminous fluxes emitted from the light source isfurther provided in the pre-deflection optical system; and the positionadjusting mechanism adjusts positions of the first reflection mirrorsafter the third optical element is moved in the optical axis directionand a focus in the main scanning direction of the beam of one colorcomponent.
 7. An optical beam scanning method comprising the steps of:preparing an optical beam scanning apparatus including one or pluralfirst optical element, plural second optical elements and plural firstreflection mirrors; emitting one or plural luminous fluxes; forming theemitted luminous fluxes to image the luminous fluxes as a line image ina direction corresponding to a main scanning direction; scanning theluminous fluxes against a scanning object in the main scanningdirection; at least, acting on the luminous fluxes for all colorcomponents by one or plural first optical elements, respectively actingon the luminous fluxes for each of color components, and imaging thescanned luminous fluxes on the scanning object; reflecting luminousfluxes emitted from the first optical elements by plural firstreflection mirrors respectively provided on an upstream side of theplural second optical elements in optical paths; and adjusting positionsof the first reflection mirrors.
 8. An image forming apparatus includingan optical beam scanning apparatus, the optical beam scanning apparatuscomprising: a light source configured to emit one or plural luminousfluxes; a pre-deflection optical system configured to form the luminousfluxes emitted from the light source to image the luminous fluxes as aline image in a direction corresponding to a main scanning direction; alight deflecting device configured to scan the luminous fluxes against ascanning object in the main scanning direction; a post-deflectionoptical system configured to at least include one or plural firstoptical elements which act on the luminous fluxes for all colorcomponents, plural second optical elements which respectively act on theluminous fluxes for each of color components, and plural firstreflection mirrors that are respectively provided on an upstream side ofthe plural second optical elements in optical paths and reflect luminousfluxes emitted from the first optical elements, the post-deflectionoptical system imaging the luminous fluxes scanned by the lightdeflecting device on the scanning object; and a position adjustingmechanism configured to adjust positions of the first reflectionmirrors.
 9. An optical beam scanning apparatus comprising: a lightsource configured to emit one or plural luminous fluxes; apre-deflection optical system configured to form the luminous fluxesemitted from the light source to image the luminous fluxes as a lineimage in a direction corresponding to a main scanning direction; a lightdeflecting device configured to scan the luminous fluxes against ascanning object in the main scanning direction; a post-deflectionoptical system configured to at least include one or plural firstoptical elements which act on the luminous fluxes for all colorcomponents, and plural second optical elements which respectively act onthe luminous fluxes for each of color components, the post-deflectionoptical system imaging the luminous fluxes scanned by the lightdeflecting device on the scanning object; and a position adjustingmechanism configured to adjust positions of the second optical elements.10. The optical beam scanning apparatus according to claim 9, whereinthe deflection surfaces provided in the light deflecting device havedifferent angles with respect to the rotation center axis for each ofthe color components.
 11. The optical beam scanning apparatus accordingto claim 9, wherein the position adjusting mechanism adjusts positionsof the second optical elements along incidence optical paths of thesecond optical elements.
 12. The optical beam scanning apparatusaccording to claim 9, wherein a third optical element which givespredetermined characteristic to the luminous fluxes emitted from thelight source is further provided in the pre-deflection optical system;and the position adjusting mechanism adjusts positions of the secondoptical elements by moving the second optical elements along incidenceoptical paths of the second optical elements after the third opticalelement is moved in the optical axis direction and a focus in the mainscanning direction of the beam of one color component.
 13. An opticalbeam scanning method comprising the steps of: preparing an optical beamscanning apparatus including one or plural first optical element andplural second optical elements; emitting one or plural luminous fluxes;forming the emitted luminous fluxes to image the luminous fluxes as aline image in a direction corresponding to a main scanning direction;scanning the luminous fluxes against a scanning object in the mainscanning direction; at least, acting on the luminous fluxes for allcolor components by one or plural first optical elements, respectivelyacting on the luminous fluxes for each of color components, and imagingthe scanned luminous fluxes on the scanning object; and adjustingpositions of the second optical elements.
 14. An image forming apparatusincluding an optical beam scanning apparatus, the optical beam scanningapparatus comprising: a light source configured to emit one or pluralluminous fluxes; a pre-deflection optical system configured to form theluminous fluxes emitted from the light source to image the luminousfluxes as a line image in a direction corresponding to a main scanningdirection; a light deflecting device configured to scan the luminousfluxes against a scanning object in the main scanning direction; apost-deflection optical system configured to at least include one orplural first optical elements which act on the luminous fluxes for allcolor components, and plural second optical elements which respectivelyact on the luminous fluxes for each of color components, thepost-deflection optical system imaging the luminous fluxes scanned bythe light deflecting device on the scanning object; and a positionadjusting mechanism configured to adjust positions of the second opticalelements.